Double stroke pistons are used in a variety of different types of industrial applications, such as for pumping heated adhesives or performing other work. Moving the piston in each direction may be accomplished by directing a pressurized fluid through a fluid valve, such as an air valve. Typically, pressurized air is directed into a piston chamber via an air valve having a valve element moveable between two positions. In the first position, the pressurized air is directed to one side of the piston within the piston chamber and in the second position the pressurized air is redirected to the other side of the piston within the piston chamber. The piston and a connected piston shaft therefore move in one direction or the other depending on the side of the piston against which the pressurized air is directed. In various prior piston pumps, the piston shaft is connected to a shifter device through a fork or other connecting member. One example is disclosed in U.S. Pat. No. 5,325,762, which is assigned to the assignee of the present invention. As the piston and shaft approach the respective first and second ends of the stroke, the shifter device is moved through magnetic force generated between magnets on the shifter device and on the fork. This causes the valve element to shift between the first and second positions. The process repeats itself at each end of the piston stroke to continuously change the direction of the piston and shaft during, for example, a pumping operation.
A continuing need for improvements in the technology related to shifting mechanisms exists. For example, some mechanisms are relatively complex, or use multiple permanent magnets, or have other needs for improvement. Further, it is desirable to ensure that the shifting mechanisms remain operative and reliable for millions of strokes in a wide variety of applications, including in some cases high temperature environments associated with pumping heated adhesives or so-called hot melt adhesives.